Perforating a Well Formation

ABSTRACT

A technique that is usable with a subterranean well includes reducing a stress on a formation in the well. The formation is perforated while the stress is reduced.

BACKGROUND

The invention generally relates to perforating a well formation.

For purposes of producing well fluid from a formation, the formation typically is perforated from within a wellbore to enhance fluid communication between the reservoir and the wellbore. In the perforating operation, a perforating gun typically is lowered downhole (on a string, for example) inside a casing (that lines the wellbore) to the region of the formation to be perforated; and subsequently, shaped charges of the perforating gun are fired to pierce the well casing and produce corresponding perforations in the formation. All other things being equal, deeper perforations typically lead to greater well productivity (i.e., more oil or gas produced per unit time per unit of extraction energy).

SUMMARY

In an embodiment of the invention, a technique that is usable with a subterranean well includes reducing a stress on a formation and while the stress is reduced, perforating the formation.

Advantages and other features of the invention will become apparent from the following description, drawing and claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a multicurve graph illustrating perforation penetration depth versus the effective stress of the formation.

FIGS. 2, 3, 4, 7 and 9 are flow diagrams depicting perforating techniques according to different embodiments of the invention.

FIG. 5 is a schematic diagram of a well depicting a system to increase pore fluid pressure to reduce the effective stress on the formation according to an embodiment of the invention.

FIG. 6 is a schematic diagram of a well depicting a multibore technique to increase pore fluid pressure to reduce the effective stress on the formation according to an embodiment of the invention.

FIG. 8 is schematic diagram of a well depicting a technique that uses thermal energy to reduce the stress on the formation according to an embodiment of the invention.

DETAILED DESCRIPTION

The depth (herein called the “perforation penetration depth”) to which a shaped charge penetrates a formation typically varies inversely with the effective stress on the formation rock.

The effective stress is the difference between the mean total stress on the reservoir rock and some multiple of the pore fluid pressure (the multiple generally being one, but perhaps slightly less than one), where the mean total stress is the average of the vertical and two horizontal components of total stress. More specifically, the mean total stress (called “sm”) may be mathematically described as follows: $\begin{matrix} {{sm} = \frac{{sv} + {sH} + {sh}}{3}} & {{Equation}\quad 1} \end{matrix}$

where “sv” represents the vertical component (i.e., the “overburden stress”) of the total stress, “sH” represents the maximum horizontal total stress component and “sh” represents the minimum horizontal total stress component.

The effective stress (called “s_eff”) may be mathematically described as follows: s _(—) eff=sm−a·Pp   Equation 2

where “Pp” represents the pore pressure, and “a” represents Biot's constant which falls in the range of zero to one (for many cases “a” equals one).

Subterranean rocks that exhibit high effective stress values tend to reduce a shaped charge's penetration effectiveness. Thus, by decreasing the formation's mean total stress and/or increasing its pore fluid pressure (i.e., reducing the effective stress), the perforation penetration depth into the formation may be improved.

FIG. 1 depicts a perforation penetration depth 2 into a formation versus an effective stress 4 on the formation for different perforating charges (depicted by the different curves 6). The effect of the formation rock stress is charge dependent, as illustrated by the different curves 6. However, as depicted in FIG. 1, increasing the effective stress on the formation from zero to 5,000 pounds per square inch (psi) reduces the perforation penetration depth by ten to forty percent. The techniques and systems disclosed herein take advantage of this perforation penetration depth versus effective stress relationship to increase the penetration depth. Thus, FIG. 1 depicts that reducing the effective stress from 5,000 psi to zero should increase the penetration depth by eleven to sixty-seven percent.

Therefore, by decreasing the formation's mean total stress and/or increasing its pore fluid pressure (i.e., reducing the effective stress), the perforation penetration depth into the formation may be improved. In accordance with some embodiments of the invention, the approach that is described herein reduces the effective stress on the formation by increasing the pore fluid pressure. In the limit, the pore fluid pressure may be increased to a value that causes the effective stress to become negative. In this state, the rock matrix is in a net tensile state, and the penetration depth may be further enhanced. Thus, the rock stress state accomplished in the techniques that are described herein may approach the stress on the rock, which is created during a hydraulic fracturing operation.

In accordance with some embodiments of the invention, the operations described herein may include a “pre-perforation” step in which a few perforations are formed in the formation so that fluid may be injected into these pores to increase the local pore fluid pressure. The increase in the local pore fluid pressure, in turn, reduces the effective stress on the formation so that perforation depth is improved. These pre-perforation steps may not be performed in other embodiments of the invention. For example, the pre-perforation steps may not be performed for an open hole completion, in accordance with some embodiments of the invention.

Referring to FIG. 2, as a more specific example, a technique 10 may be used for purposes of increasing the perforation penetration depth. Pursuant to the technique 10, the effective stress is reduced (block 12) on a well formation; and while the effective stress is reduced, the formation is perforated, as depicted in block 14. The reduction of the effective stress on the well formation may be temporary in nature. For example, if the nearby pore fluid pressure is increased by increasing fluid pressure inside the wellbore, the time in which the nearby pore fluid pressure is elevated may depend on the permeability of the formation. Thus, the initial pressurization establishes a pressure gradient between the nearby pore fluid pressure and the far field pore fluid pressure; and the permeability of the formation establishes the time for the nearby pore fluid pressure to once again equal the far field pore fluid pressure. The perforating operation is performed during the time of reduced effective stress, a time in which the nearby pore fluid pressure comes close to or even exceeds the mean total stress of the formation.

As a more specific example, FIG. 3 depicts a technique 20 in accordance with an embodiment of the invention. The technique 20 includes reducing (block 22) the effective stress on a formation caused by the difference between the mean total stress and the pore fluid pressure by increasing the pore fluid pressure. While the pore fluid pressure is increased, the formation is perforated, as depicted in block 24.

As a more specific example of a way to increase pore fluid pressure, FIG. 4 depicts a technique 30 in which preliminary perforations are created (block 34) inside an interval of a wellbore to establish communication between a reservoir and the wellbore. This interval is subsequently sealed off, as depicted in block 36. Next, pressure is applied to the wellbore inside the interval to increase the pore fluid pressure, as depicted in block 38. Finally, pursuant to the technique 30, the formation is perforated (block 40) in the interval while the fluid pore pressure remains elevated.

It is noted that many different techniques may be used to increase the pressure inside the interval. For example, in some embodiments of the invention, fluid may be pumped from the surface of the well into the interval until the desired pressure (a pressure near the mean total stress, for example) is obtained. The fluid flow from the surface then stops, which means the pressure inside the interval gradually reduces due to the permeability of the formation. However, in other embodiments of the invention, fluid may be continually pumped into the interval to negate the fluid and pressure loss inside the interval and thus, maintain the constant fluid pressure inside the interval nearby. Thus, many variations are possible and are within the scope of the appended claims.

Referring to FIG. 5, an exemplary system 50 to perform at least a portion of the technique 30 may include a string 60 that extends into a wellbore 54. As depicted in FIG. 5, the wellbore 54 may be lined by a casing string 56. However, the technique that is described herein may be equally applied to uncased wellbores, in other embodiments of the invention.

The string 60 may include a selectable perforating gun (not depicted in FIG. 5), which may be used to from preliminary perforations 72 in a formation 51 from which production is to occur. However, in other embodiments of the invention, the preliminary perforations 72 may be formed by a perforating gun that is lowered downhole in a prior run. Alternatively, in other embodiments of the invention, the perforations 72 may be pre-existing as part of an older well. Thus, many variations are possible and are within the scope of the appended claims.

The preliminary perforations 72 establish fluid communication flow paths 74 into the formation 51. Thus, by the formation of the preliminary perforations 72, fluid communication is established between the wellbore 54, through the casing string 56 and into the formation 51.

The string 60 includes at least one device to form an annular seal, such as a packer 64, for example. In some embodiments of the invention, the packer 64, when set, forms the upper boundary of an isolated interval 80 of the well. The lower end of the interval 80, in turn, may be formed by a sealing device, such as a bridge plug 90, for example. As an example, the bridge plug 90 may be set at the lower end of the interval 80 in a prior run. However, in other embodiments of the invention, the bridge plug 90 may be set in place by a setting tool that is disposed at the lower end of the string 60. Alternatively, the bridge plug 90 may be replaced by a packer that is part of the string 60. Therefore, many different seal arrangements may be used to form the isolated zone 90 in the various embodiments of the invention.

Thus, in some embodiments of the invention, the upper packer 64 is set to establish the isolated interval 80 after the formation of the preliminary perforations 72. After the isolated interval 80 is created, fluid may then be pumped from the surface of the well through the central passageway of the string 60. The pumped fluid exits the central passageway of the string 60 through radial ports 70 (for example) into the isolated interval 80. Thus, by pumping fluid from the surface of the well, the region of the well inside the isolated interval 80 is pressurized. The resultant fluid pressure is communicated into the formation 51 to increase the nearby pore fluid pressure. This increase in nearby pore fluid pressure, in turn, reduces the effective stress on the formation 51 near the wellbore 54, i.e., the portion of the formation 51 in which perforating is to occur.

After the increase in fluid pressure, shaped charges of a perforating gun 84 (of the string 60) are fired to create corresponding extended depth perforations 86 into the formation 51. Depending on the particular embodiment of the invention, the perforating gun 84 may be fired by any of a number of different mechanisms, such as tubing conveyed pressure, an inductive coil, an electrical wire, etc. As depicted in FIG. 5, the extended depth perforations 86, due to the reduced effective stress, extend deeper into the formation 51 than the preliminary perforations 72.

As set forth above, the firing of the shaped charges of the perforating gun 84 occurs during the time interval in which the nearby pore fluid pressure is elevated (near or exceeding the mean total stress, for example).

The nearby pore fluid pressure may be increased using other techniques, in accordance with other embodiments of the invention. For example, FIG. 6 depicts a system 100 to increase the nearby fluid pore pressure in a formation 104 in accordance with an embodiment of the invention. Unlike the system 50 that is depicted in FIG. 5, the system 100 does not use an increased or high wellbore pressure inside a wellbore 112 (a lateral wellbore in this example) in which extended depth perforations 170 are formed. Instead of pressurizing the wellbore 112, an adjacent wellbore, such as an adjacent lateral wellbore 110, is used as an injector to pressure up the reservoir and increase the local pore fluid pressure near the wellbore 112.

More specifically, in accordance with some embodiments of the invention, the wellbores 110 and 112 share a common vertical wellbore 102. However, it is noted that the arrangement that is depicted in FIG. 6 is for purposes of example only. Thus, in other embodiments of the invention, the wellbores 110 and 112 may be located in entirely different wells that, although in hydraulic communication through the formation, do not share a common bore or section.

Referring to the specific arrangement that is depicted in FIG. 6, a string 120 may be inserted into the wellbore 110 for purposes of pressurizing the reservoir and increasing the pore fluid pressure near the wellbore 112. The string 120 may include, for example, a sealing element, such as a packer 122, for purposes of forming one end of an isolated interval 114. Another end of the isolated interval 114 may be formed by another seal 124. As examples, the seal 122 may be a packer that is set to form an annular seal between the string 120 and the wall of the wellbore 110; and the seal 124 may be a bridge plug. Furthermore, in some embodiments of the invention, the bridge plug may be run in separately from the string 120; or alternatively, the bridge plug may be run and set by the string 120. Thus, many variations are possible and are within the scope of the appended claims.

After the seals 122 and 124 are in place to form the isolated interval 114, radial ports 130 of the string 120, which are located inside the interval 114, may be used to communicate pumped well fluid from the well surface for purposes of pressurizing the isolation interval 114 and thus, pressurizing the formation near the wellbore 112. Prior to this fluid pressurization of the interval 114, one or more preliminary perforations 129 may be formed in the interval 114 to enhance fluid communication between the reservoir and the isolated interval 114.

The pressurization of the fluid inside the interval 114 increases the nearby pore fluid pressure of the wellbore 112. During this time interval of elevated pore fluid pressure, shaped charges of a perforating gun 160 (located in the wellbore 112) may be fired to form extended depth perforations 170 that extend into the formation 104 from the wellbore 112 prior to the pressurization of the interval 114. As an example, the perforating gun 160 may be part of another string 150 that is lowered downhole inside the wellbore 102 and inside the lateral wellbore 112. In some embodiments of the invention, a sealing element 154, such as a packer, may form a seal between the string 150 and the interior wall of the wellbore 112.

To summarize the technique used to form the extended depth perforations 170, the strings 120 and 150 are run inside the well. The preliminary perforations 129 may be subsequently formed in the wellbore 110; or, alternatively, the preliminary perforations 129 may be formed prior to the running of the string 120 into the well. The string 120 is then used to form the sealed interval 114 so that fluid pressure may be increased inside the interval 114 to increase the pore fluid pressure near the wellbore 112. While the pore fluid pressure near the wellbore 112 is elevated (near or exceeding the mean total stress, for example), the perforating gun 160 fires its shaped charges to create the extended depth perforations 170.

Referring to FIG. 7, to generalize, in accordance with an embodiment of the invention, a technique 180 includes creating preliminary perforations inside an interval of a first wellbore to establish communication between a reservoir and the first wellbore, as depicted in block 184. Next, the interval is sealed off, as depicted in block 186. Pressure is then applied to the first wellbore inside the interval to increase the pore fluid pressure near another second wellbore, as depicted in block 188. Subsequently, the formation is perforated from the second wellbore while the pore fluid pressure remains elevated, as depicted in block 190.

It is noted that an annular seal (to form the above-described isolated interval) may not be formed in other embodiments of the invention. For example, in accordance with some embodiments of the invention, the wellbore may be pressurized using a heavier fluid so that the hydrostatic head of the fluid may be relied on rather than an annular seal to isolate the region of the well in which the perforating occurs.

Other techniques may be used to reduce the effective stress on the formation, in other embodiments of the invention. For example, referring to FIG. 8, in accordance with some embodiments of the invention, a wellbore system 200 includes a string 210 that, in turn, includes one or more heating elements 220 for purposes of altering a pressure balance between the formation rock matrix and the nearby pore fluid. More specifically, the string 210 includes a perforating gun 214 that is lowered downhole inside a wellbore 202 (lined by a casing string 104 in this example) to a position in which extended depth perforations 230 are to be formed. The heating elements 220 may be integrated among the perforating charges of the perforating gun 214, above the perforating gun 214 or below the perforating gun 214, depending on the particular embodiment of the invention. Furthermore, the string 210 may include a sealing element, such as a packer 212, to form a seal between the outside of the string 210 and the casing 204.

Therefore, when the extended depth perforations 230 are to be formed, the perforating gun 214 is lowered downhole on the string 210 until the perforating gun 214 reaches the proper position. Afterwards, the packer 212 is set and the heater elements 220 is activated to heat up the formation to alter the pressure balance between the rock matrix and the pore fluid. After the pressure balance has been significantly altered, the shaped charges of the perforating gun 214 may then be fired to form the extended depth perforations 230.

Referring to FIG. 9, thus, in accordance with some embodiments of the invention, a technique 250 includes applying (block 252) thermal energy inside an interval of a wellbore to alter the pressure balance between a formation rock matrix and pore fluid due to differences in thermal expansion coefficients. Next, the formation is perforated (block 256) in the interval while the pressure balance remains altered.

The above-described application of thermal energy assumes that the thermal expansion coefficient of the fluid is greater than the thermal expansion coefficient of the formation. By knowing these thermal expansion coefficients, the temperature needed to be achieved may be determined. Furthermore, knowledge of the relevant heat capacities and heat transfer rates enables determination of the energy and power requirements, respectively.

The thermal energy may be applied using other arrangements, in other embodiments of the invention. For example, the string 210, instead of containing the heater elements 220, may instead communicate a heated fluid from the surface into an isolated zone to be perforated. Alternatively, a chemical reaction may be used to generate the thermal energy. Thus, many variations are possible and are within the scope of the appended claims.

Other techniques and systems to reduce the effective stress on the formation during perforating to extend perforation depth are possible and are within the scope of the appended claims.

While the present invention has been described with respect to a limited number of embodiments, those skilled in the art, having the benefit of this disclosure, will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention. 

1. A method usable with a subterranean well, comprising: reducing a stress on a formation in the well; and while the stress is reduced, perforating the formation.
 2. The method of claim 1, wherein the stress comprises an effective stress on the formation.
 3. The method of claim 1, wherein the stress is caused by a difference between a mean total stress of the formation and a pore fluid pressure of the formation.
 4. The method of claim 1, wherein the act of reducing comprises increasing a pore fluid pressure.
 5. The method of claim 4, wherein the act of reducing further comprises temporarily increasing the pore fluid pressure during a time interval, wherein the act of perforating occurs during the time interval.
 6. The method of claim 5, wherein the time interval is a function of a permeability of the formation.
 7. The method of claim 1, wherein the act of reducing comprises: sealing off an interval of the well containing perforations; and pressurizing the interval.
 8. The method of claim 7, further comprising: perforating the formation to form the perforations.
 9. The method of claim 1, further comprising: forming perforations in the formation prior to the act of perforating.
 10. The method of claim 1, wherein the act of reducing comprises: pressuring the well with a heavy fluid to form a hydrostatic head to isolate a region of the well.
 11. The method of claim 1, wherein the perforating occurs in a first wellbore and the act of reducing is performed at least partially in another second wellbore.
 12. The method of claim 11, wherein the act of reducing further comprises pressurizing the second wellbore to reduce the stress of the formation near the first wellbore.
 13. The method of claim 1, wherein the act of reducing comprises applying thermal energy to the formation.
 14. The method of claim 13, wherein the act of applying thermal energy alters a pressure between the formation and the pore fluid.
 15. A system usable with a subterranean well, comprising: a first tool to reduce a stress on a formation; and a perforating tool to, while the stress is reduced, perforate the formation.
 16. The system of claim 15, wherein the stress comprises an effective stress on the formation.
 17. The system of claim 15, wherein the stress is caused by a difference between a mean total stress of the formation and a pore fluid pressure of the formation.
 18. The system of claim 15, wherein the first tool is used to increase a pore fluid pressure on the formation.
 19. The system of claim 18, wherein the first tool is used temporarily increase the pore fluid pressure during a time interval so that the perforating tool may form the perforations in the formation during the time interval.
 20. The system of claim 19, wherein the time interval is a function of a permeability of the formation.
 21. The system of claim 15, wherein the first tool and the perforating tool are part of a string, the string further comprising a sealing device to seal off an interval of the well containing the perforations so that the interval may be pressurized.
 22. The system of claim 21, wherein the perforating tool is adapted to form the perforations during the existence of the seal.
 23. The system of claim 15, wherein the first tool comprises a heater element.
 24. The system of claim 15, wherein the first tool and the perforating tool are part of a string. 